Finned strainer

ABSTRACT

The present invention relates to filters used to remove debris from water being sucked into a piping system. It has particular application use in nuclear power plants, which, after a loss of coolant accident, must pump cooling water back into the reactor core from a collection sump. This water may contain various types of debris that must be removed before the water is sent back into the reactor cooling system. There are restrictions on the allowable pressure drop across the strainer and the space available for installing this equipment. The finned strainer of the present invention addresses these issues while maximizing the quantity of debris filtered from the water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/666,896, filed on Jan. 30, 2008, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention pertains to the field of filters used to removedebris from water being sucked into a piping system. More particularly,the present invention relates to the field of 5 filters used in nuclearpower plants.

BACKGROUND

Nuclear plants have various safety systems to ensure that the nuclearfuel in the reactor core remains cooled in all credible accidentscenarios. One such scenario is a “loss of coolant accident,” in whichan external pipe is postulated to break, allowing a large amount ofwater to escape from the reactor cooling system. This water may dislodgesolid debris from neighbouring pipes or other reactor structures. Thewater, along with some of the dislodged debris, will flow to the lowestparts of the reactor building into a sump. Plants are equipped withsafety systems that pump water from the sump back into various reactorcooling systems. Strainers on the pump intakes ensure that any debrislarge enough to clog equipment in these systems is prevented from 15entering. Depending on the type of debris, the first layer to deposit onthe strainer may form a finer filter than the underlying screen, andcatch many smaller particles.

Strainers must have enough screen area that the debris layer on thestrainer is not too thick to cause unacceptably high restriction toflow. Strainers must also be as small as possible to fit into theavailable space. Therefore compactness, i.e., accommodating the mostscreen area in the smallest volume, is important.

Conventional strainers in many nuclear plants are simple box-typedevices mounted over the pump intakes. Newer more advanced strainersoften have an irregular surface to increase the surface area.

This background information is provided for the purpose of making knowninformation believed by the applicant to be of possible relevance to thepresent invention. No admission is necessarily intended, nor should beconstrued, that any of the preceding information constitutes prior artagainst the present invention.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention there is provided astrainer for filtering debris from a fluid comprising an elongatedheader defining an enclosed flowpath, having an outlet in fluidcommunication with a suction source and a plurality of inlet aperturesdisposed along the length of said flowpath, said flowpath exhibiting apressure drop in the direction of fluid flow; a strainer elementdisposed in each said inlet aperture for straining debris from fluidentering said flowpath; and a flow controlling device for maintainingsubstantially uniform fluid flow through strainer elements located atdifferent positions along said flowpath.

In accordance with another aspect of the invention, the flow controllingdevice comprises an orifice for producing a pressure drop between aninlet aperture and the flowpath located at a position closer to saidsuction source that is greater than the pressure drop between an inletaperture and said flowpath located at a position farther from saidsuction source. The orifice can be in the form of a nozzle foraccelerating the fluid entering said flowpath in a directionsubstantially parallel thereto and can be formed in a baffle disposed insaid header, the baffle defining a collection channel enclosing aplurality of apertures.

In accordance with another aspect of the present invention, the headerhas a generally planar side-wall and the inlet apertures are a series ofsubstantially parallel slots formed in the side-wall in a directiontransverse to said flowpath. The strainer elements can be in the form offlat-surface fins projecting outwardly from the apertures in the planarside-wall.

In accordance with another aspect of the present invention, there isprovided a strainer for filtering debris from a fluid comprising aheader defining an enclosed volume and having an outlet in fluidcommunication with a suction source, said header having a plurality ofinlet aperture slots formed therein, a fin-like strainer elementprojecting outwardly from each aperture slot for straining debris fromsaid fluid, each said strainer element comprising a perimeter frame anda pair of fluid permeable screens fixed thereto in opposed spacedrelation, and at least one fluid flow channel therebetween in fluidcommunication with said enclosed volume through a marginal side edge ofsaid frame and said aperture slot. The fluid permeable screens can beformed of perforated metal sheet or mesh. A corrugated metal spacer canbe disposed between the fluid permeable screens for maintaining saidfluid permeable screens in spaced relation, and a plurality of flowchannels can be defined between said corrugated metal spacer and saidfluid permeable screens. The perimeter frame can be impermeable to fluidexcept at said one marginal side edge.

In accordance with another aspect of the present invention, the fluidpermeable screens are each formed of corrugated metal mesh having aplurality of parallel peaks and valleys, said screens being maintainedin opposed spaced relation by contact at alternating peaks and defininga plurality of said flow channels therebetween.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an isometric view of a strainer module for connection to apump intake in accordance with an embodiment of the present invention.

FIG. 2 is an exploded view of the strainer module shown in FIG. 1;

FIG. 3 is a cut-away isometric view of a strainer module mounteddirectly onto a sump in accordance with an embodiment of the presentinvention;

FIG. 4 is a cut-away isometric view of a section of a flat-surface finin accordance with an embodiment of the present invention;

FIG. 5 is a cut-away isometric view of a section of a corrugated-surfacefin in accordance with an embodiment of the present invention;

FIG. 6 is an exploded view of a corrugated-surface fin in accordancewith an embodiment of the present invention;

FIG. 7 is an isometric section view of flow equalization devices inaccordance with an embodiment of the present invention; and

FIGS. 8A, 8B, 8C, 8D, 8E and 8F depict perspective views of acorrugated-surface fin in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, the strainer module of the present inventioncomprises elongated header 3 that defines an internal fluid flowpaththat is in fluid communication with a suction source through pump intakepipe 2 which may be located in the floor or wall through one or moreconnection(s) 1. Header 3 has a generally planar sidewall with aplurality of inlet apertures 9 in the form of a series of substantiallyparallel elongated slots disposed along the length of the header toaccommodate fins 4. The inlet apertures are oriented in a directiontransverse to the fluid flowpath within header 3. Strainer elements inthe form of hollow flat-surface fins 4 may be mounted on the sides (asshown in FIG. 1), top, or bottom of header 3 and project outwardly frominlet apertures 9. Fins 4 may have a uniform or variable spacing and arelocated by mounting frames 5 and braces 6. In a preferred embodiment,fins are easily removable, using a pins 10 and bolts 11, but they mayalso be permanently attached to header 3.

Water enters the strainer through fluid permeable screens 7 on thesurface of the fins 4, leaving debris on the screens. Water then flowsthrough the fluid flow channel in hollow core 8 of the fin 4 towardsheader 3. Various portions of header 3, particularly the portion betweenfin slots 9, may be made using fluid permeable material to increase thefiltration area. Header 3 may have one or more baffle plates 12 toprovide structural support for the sides to resist high suctionpressures. Baffle plates 12 have large holes 13 to ensure flow velocityin the fluid flow channel in header 3 is the same above and below thebaffle.

The ends of each header 3 have flanges 14 that allow adjoining modulesto be attached together. Modules may be attached together, or they maybe independently mounted with seals between modules. Mounting frames 5may be provided under the module. Mounting frames 5 haveadjustable-height mounts 15 that allow the device to be installed onfloors that are not level.

FIG. 3 shows an alternative embodiment of the present invention. Thisembodiment is useful for situations where there is a pre-existing sump45 with a cover 46 (which could be pre-existing or installedspecifically to support the strainer modules) and a pomp intake 47inside the sump 45. Strainer module 48 comprises fins 49 mounted in aframe 50 with appropriate bracing. For simplicity, only one module hasbeen shown in FIG. 3. If required, a plurality of strainer modules 48can be mounted in a similar manner over sump 45.

Flow enters fins 49 in the same manner as described above in relation tothe embodiment shown in FIGS. 1 and 2, but then flows directly into sump51, and then into the pump intake 52. Pump intake 52 may be modified toreduce inlet losses. No separate collection header is required for thisarrangement, because sump 51 itself performs this function. Undesirablebypass flow between the sump cover 46 and the module frame 50 can beprevented using close tolerances between mating parts, or using wiremesh gaskets or any other suitable type of seal, such as item 25 shownin FIG. 5 on the edge of the fin. Suitable portions of the module frame50 or sump cover 46 may be made from perforated metal sheet to increasethe filtration area. To make use of some of the volume of sump 51, thestrainer module can be recessed, either partially or completely, intothe sump below the level of the floor. In such a case, the frame 50 ofthe module would extend down from the floor level to the bottom of themodule in order to prevent flow from bypassing the filtration elementsof the strainer.

Air ingestion can be prevented by ensuring that there is a sufficientheight of water above the strainer. In the alternative, a horizontalcover (not shown) can be added over the fins. This cover allows the finsto be closer to the water surface without ingesting air or causinghollow-core vortices.

Various types of bracing, such as those shown in FIGS. 1, 2 and 3, areused to ensure that the strainer is sufficiently rigid to resist thepredicted seismic and pressure loads. In addition, external bracing,such as indicated by reference numeral 6 in FIG. 2, may also be placedbetween fins.

For all applications, it is desirable to optimize the design for thetype and quantity of debris that the strainer is required to handle. Twobasic factors need to be considered: the filtration area required, andthe potential volume of debris that must be accommodated within thestrainer. The number of fins is determined by the required filtrationarea, and then fin spacing can be varied to ensure that there issufficient space between fins to accommodate the potential debrisvolume. The strainer module is advantageously fabricated in a size thatis easily manageable and can be moved into place without interferencewith surrounding equipment. In addition, a complete strainer assemblymay contain as many strainer modules as necessary.

Two types of fins that may be incorporated in the apparatus of thepresent invention are discussed below in relation to FIGS. 4, 5, 6 and8.

Flat-Surface Fin

Referring now to FIG. 4, in accordance with one aspect of the presentinvention, flat-surface fins, have a pair of planar fluid permeablescreens 16 of perforated metal sheet on each side of the fin in opposedspaced relation. Screens 16 filter debris from the water entering thestrainer. The two screens 16 are separated by a corrugated metal spacer17. Spacer 17 provides stiffness and strength, and also forms flowchannels between screens 16 to header 3. The edges of screens 16 areenclosed by perimeter frame 18. The flow channels between screens 16 areopen to fluid communication with the enclosed flowpath in header 3through the marginal side edge of frame 18 that fits into aperture 9 ofheader 3. Frame 18 also adds significantly to the structural strength ofthe fin.

If the application requires smaller filtration holes than are achievableusing standard perforated metal mesh, a layer of fine wire mesh may belaminated onto the surface of perforated metal screens 16 of the fin.

The advantages of the fin construction shown in FIG. 4 includesimplicity of manufacture and minimal internal volume.

Corrugated-Surface Fin

Referring now to FIGS. 5, 6 and 8, in accordance with another aspect ofthe present invention, corrugated-surface fins are constructed of twolayers of perforated metal screen 19 that have been corrugated toincrease their exposed surface area. Debris filtered out of waterentering the strainer is deposited on these corrugated surfaces.

The corrugations provide a number of advantages. The large increase infiltration surface area over a flat screen is a very significantadvantage for thin debris layers, which often pose more of a problemthan more porous thick debris layers. The increased area reduces therestriction to flow entering the strainer by decreasing the watervelocity through the screen and reducing the thickness of debris(because it is spread over a larger area). The “peaks” of thecorrugations also reduce the pressure drop by tending to encourage alocally non-uniform debris bed. Even with layers of debris that arethicker than the height of the corrugations, there can be a significantbenefit, because fine particulates often migrate through the debris bedand concentrate near the filtration surface, causing a thin relativelyimpermeable layer at the surface. The resistance of this thin layer toflow entering the strainer is reduced with the larger screen areaachieved by the corrugations.

Another important feature of this design is that the corrugated screenscan be made strong enough to be relied on as the sole structural elementin the fin. Moreover, the screen can also be formed using relativelythin gauge material. This minimizes the amount of material required tomake a fin, saving cost and making the fins easier to handle because oftheir reduced weight.

The corrugated metal mesh screens have a plurality of parallel “peaks”and “valleys” and are positioned in opposed spaced relation such thatalternating peaks in one screen are in tip-to-tip contact withalternating peaks in the opposed screen. This configuration forms hollowinternal channels for fluid entering the strainer to flow towards thecollection header. These flow channels are unobstructed and can be madelarge enough to offer minimal restriction to flow.

The internal volume of the design is minimized, therefore maximizing thespace outside the strainer to collect debris.

As shown in FIG. 5, the perimeter frame about the fluid permeablescreens can comprise flat bar 20 to seal the edges parallel to thecorrugations and to provide strong attachment points 21 for bracing 6and fin attachment hardware 10 and 11. These edges can also be sealedwith perforated metal screen 22 to further increase the filtrationsurface area.

The perimeter frame about the fluid permeable screens can also compriseperforated metal caps 23 to seal the ends of the corrugations. Theadvantage of this type of end cap is that it adds to the perforatedscreen area and does not restrict flow access to the space between thefins. In the embodiment shown in FIGS. 6 and 8, end cap 40 is formedfrom a channel, which is then welded over the end of the fin. Theadvantage of this type of end cap is that it is simple to manufactureand adds significantly to the strength of the fin. It can be fully orpartially perforated if extra screen area is needed.

The marginal side edge of the perimeter frame at the edge of the finfitting into the header is adapted to a rectangular cross-section to fitthe rectangular slots 9 in the header. This can be done using a toothedstrip of perforated metal 24, with the edge sealed into the collectionheader with flexible metal strips 25 as shown in FIG. 5. FIGS. 6 and 8also shows a simpler design of a cap 41 for the portion of the perimeterframe at the header end of the fin, Cap 41 is formed from a channel thatis welded over the ends of the corrugated metal mesh sheets. Cap 41 haslarge openings through which flow 43 from the channels between thecorrugations communicates with apertures 9 and the flowpath enclosed inheader 3. The sides of the end cap provide surfaces to attach seals 42,which ensure a good fit of the fin into the header.

Flow Equalization

Reasonably uniform flow is desirable to prevent formation of hollow-corevortexes and to ensure that debris depositing onto the strainer is notpacked too densely. If the flow concentrates at one spot, debris willquickly build up at this spot in a very dense mat, raising the flowresistance enough that flow will enter at an adjacent spot, causing adense bed to build up there also. If unrestrained, this can progressthroughout the whole strainer, causing a much higher pressure loss thanif the debris had built up uniformly.

In a further embodiment of the present invention shown in FIG. 7, theflow entering at various points along the length of the header iscontrolled using flow-balancing devices that boost the pressure insidethe header. As fluid flows along the header, frictional pressure dropand acceleration pressure drop cause decreasing pressures closer to thesuction end of the header (i.e., in the direction of fluid flow). Thiswould normally provide more driving pressure for flow entering the fins,causing somewhat non-uniform flow (more flow entering fins closer to thepump intake, or suction end). To ensure that water entering any fin issubjected to the same driving pressure differential, calibrated flowbalancing devices are added, which provide greater restriction for finscloser to the suction end of the strainer than at the far end.

In accordance with a further preferred embodiment of the presentinvention, the flow-balancing devices provide flow restriction in apartially reversible fashion. Thus, the energy required to acceleratethe flow through the flow-balancing devices is converted first tokinetic energy as a jet of water in the header pointed in the directionof flow towards the suction end. The momentum of this jet is used toboost the pressure in the header in a manner that partially offsets theupstream friction and acceleration losses. This pressure boost reducesthe amount of pressure imbalance along the length of the header. Asubstantially uniform flow can be achieved at the same time as providinga lower overall pressure loss.

FIG. 7 shows the details of this embodiment in which all but one finhave been removed for clarity. Flows at various locations areillustrated with arrows.

Fluid flow 25 enters fin-like strainer element 32 through perforatedscreens, passes through the internal channels formed by the corrugationswhich are in fluid communication with header 35 through end cap 41 (seeFIGS. 6 and 8) and slot 26. Flow from neighbouring fins similarly entersheader 35 through slots 27 and 28. A relatively narrow collectionchannel 38 inside header 35 is defined by vertical baffle 31 runninginside outer wall 36 of the header 35. Flow from the collection channelis accelerated through an orifice 33 forming a jet 39 that joins theflow 29 from upstream fins. Because the velocity of the jet 39 issubstantially parallel to the velocity of the main flow 29, the pressureat downstream flow 30 is raised over that if the water were injectedperpendicular to the main flow. Furthermore, because the velocity of thejet 39 is greater than the velocity of the main flow 29, momentum isadded to the main flow, which boosts pressure at flow 30 over that thatwould exist if the flow velocity of jet 39 were the same as that of mainflow at 29. In addition, the orifice 33 provides a smooth contractionfor the flow so that there is minimal energy loss in creating the jet39.

The pressure in the main header 35 drops as one moves closer to the pumpintake because of friction and acceleration pressure drops. Thedifferential pressure across orifices closer to the pump intake aretherefore greater than across orifices farther away. In order to balancethe flows entering the main header, the width of each orifice 33, 34 isselected so that the pressures upstream of all orifices, e.g., througheach of the collection channels 38, 37 are equal. Substantially equalpressure can be achieved by providing that orifices closer to the pumpintake have smaller flow area than orifices farther from the pump intakewith the result that substantially uniform fluid flow is maintainedthrough strainer elements located at different positions along theflowpath in header 3.

A flow control device in the form of an appropriately sized and shapedorifice can be provided for individual collection channels each of whichencloses a plurality of apertures (as shown in FIG. 7), or in thealternative, can be provided for each individual inlet aperture.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of this application.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A filtering apparatusfor a fluid intake to a collection header of a nuclear power generationfacility, the filtering apparatus comprising: a perimeter frame;opposing filtering screens mounted to said perimeter frame, whereinopposing edges of the opposing filtering screens are spaced apart by theperimeter frame; and a spacer defining an enclosed volume between atleast one of said opposing filtering screens and said spacer, saidspacer configured for maintaining said spacer in spaced relation withsaid at least one opposing filtering screen, and for maintaining saidopposing filtering screens in fixed spaced relation, said spacer andsaid at least one opposing filtering screen defining a plurality ofnon-intersecting flow channels between said spacer and the at least oneopposing filtering screen, the plurality of non-intersecting flowchannels extending through said enclosed volume; wherein the perimeterframe maintains the opposing filtering screens in spaced relationshipwith each other; wherein said opposing edges of the opposing filteringscreens are fixed to a marginal side edge of said perimeter frame, and across-section of said marginal side edge is configured to fit a slot inthe collection header; wherein the marginal side edge defines aplurality of openings; wherein each flow channel of the plurality ofnon-intersecting flow channels is configured to evacuate the fluidthrough a counterpart opening of the plurality of openings; and whereinthe filtering apparatus is configured for fluid flow to enter througheach opposing filtering screen, follow one flow channel of the pluralityof non-intersecting flow channels formed by the at least one opposingfiltering screen and the spacer, and evacuate the one flow channel tothe collection header through one counterpart opening of the pluralityof openings corresponding to the one flow channel of the plurality ofnon-intersecting flow channels.
 2. The filtering apparatus of claim 1wherein said opposing filtering screens are is formed of perforatedmetal sheet.
 3. The filtering apparatus of claim 1, wherein saidopposing filtering screens are formed of wire mesh.
 4. The filteringapparatus of claim 1, wherein said spacer comprises a metal plate. 5.The filtering apparatus of claim 4, wherein said metal plate comprises aperforated metal plate that defines an inclined surface, relative to theopposing filtering screens.
 6. The filtering apparatus of claim 4,wherein said spacer defines one or more peaks formed by converging metalsurfaces.
 7. The filtering apparatus of claim 6, wherein said spacerdefines a plurality of alternating peaks and valleys.
 8. The filteringapparatus of claim 7, wherein said spacer comprises a corrugated plateopposed to the at least one opposing filtering screen in reinforcingrelationship.
 9. The filtering apparatus of claim 1, wherein said spacerprovides reinforcement for resisting seismic loads.
 10. The filteringapparatus of claim 1, wherein the spacer is connected to the perimeterframe.